234 TRILOBITA : CHAP. 
have so far been found in 136 species of Trilobites belonging to 
39 genera ranging from Lower Cambrian to Carboniferous. 
A “metastoma ” or lower lip plate (Fig. 142, Hp) is found just 
behind the hypostome in Zriarthrus, but has not been noticed 
in any other genus. Between the hypostome and the metastoma 
hes the mouth. 
The segments of the thorax are free, and their number varies 
from two in Agnostus (Fig. 146) to twenty-six in Harpes (Fig. 
150, A). In the Trilobites confined to the Cambrian period the 
number (except in the Agnostidae) is usually larger than in the 
genera found in the Ordovician and later periods. Owing to the 
free thoracic segments many Trilobites were able to curl up some- 
what after the manner of a Wood-louse (Figs. 137, D, 138). 
The axial part of each thoracic segment is more or less con- 
siderably arched. Usually it consists of three parts: (.) the 
largest part (Fig. 137, C, a), called the ring, is band-like in 
form, and is always visible whether the Trilobite is extended or 
coiled up; (ii.) in front of the ring is a depressed, groove-like 
part (Fig. 137, C, >) separating it from (iii.) the articular portion 
(c) which is convex in front and extends beneath the ring of the 
preceding segment; this part is only visible when the Trilobite 
is coiled up or when the segments are separated. In some few 
genera the axial part consists of a simple arched band without 
either a groove or a specially modified articular portion. The 
pleurae (Fig. 137, A, 7, C, d-f) are fixed firmly to the axis, and 
have the form of narrow bands with the ends rounded, obtuse, 
pointed, or spinose. In a few cases the pleurae have a plain 
surface; but usually they possess either a ridge or a groove: 
(Fig. 137, C, g); the former is generally parallel to the margins 
of the pleura, the latter is generally oblique, being inclined 
backwards from the axis. Sometimes in front of the ridge 
there is a small groove. On the ventral surface each pleura 
shows, at its outer extremity, a reflexed margin or doublure. 
At some distance from the axis the pleurae are bent downwards 
and backwards. The point where this bend occurs is called the 
“fulerum” (e); it divides the pleura into an internal and an 
external part: the internal part (d-e) is flat or slightly convex, 
and just touches the front and back margins of the adjacent 
pleurae; the external part (¢-/) may be (i.) narrower than the 
internal part, so that it is separated from the previous and 
